feat: comprehensive Wino Mail updates

- ReaderView: complete rewrite with CSS styling, headers, attachments, zoom, find, dark mode, image blocking
- Categories: DAO + UI tree widget with colors, nested categories, assignment
- Rules engine: DAO + evaluation + actions (move, mark read, flag, delete, category, forward)
- Templates: DAO + editor + manager with variables support
- Signatures: DAO + manager + auto-per-account + manual selector in compose
- ComposeView: signature combo, template selector, auto-signature on new email
- MainWindow: frameless with rounded corners, 1px border, resize from edges, no status bar
- Database: new tables (Category, MailCategory, Rule, Template, Signature) with indexes

icons.qrc

@@ -0,0 +1,21 @@
+<RCC>
+    <qresource prefix="/icons">
+        <file alias="app.svg">resources/icons/app.svg</file>
+        <file alias="sync.svg">resources/icons/sync.svg</file>
+        <file alias="settings.svg">resources/icons/settings.svg</file>
+        <file alias="minimize.svg">resources/icons/minimize.svg</file>
+        <file alias="maximize.svg">resources/icons/maximize.svg</file>
+        <file alias="restore.svg">resources/icons/restore.svg</file>
+        <file alias="close.svg">resources/icons/close.svg</file>
+        <file>resources/icons/SVG/Outline/Essentional, UI/Trash Bin Minimalistic.svg</file>
+        <file>resources/icons/SVG/Outline/Messages, Conversation/Paperclip.svg</file>
+        <file>resources/icons/SVG/Bold Duotone/Messages, Conversation/Plain.svg</file>
+        <file>resources/icons/SVG/Outline/Files/File Text.svg</file>
+        <file>resources/icons/SVG/Bold/Text Formatting/Text Bold.svg</file>
+        <file>resources/icons/SVG/Outline/Text Formatting/Text Italic.svg</file>
+        <file>resources/icons/SVG/Outline/Text Formatting/Text Underline.svg</file>
+        <file>resources/icons/SVG/Linear/Arrows Action/Square Top Down.svg</file>
+        <file>resources/icons/SVG/Linear/Arrows Action/Square Top Up.svg</file>
+        <file>resources/icons/SVG/Linear/Essentional, UI/Close Square.svg</file>
+    </qresource>
+</RCC>